The present invention relates to electrolytic cells comprising polymeric film composition electrodes and separator membranes and to a method of economically making such cells. In particular, the invention relates to rechargeable lithium battery cells comprising an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solutions of a lithium compound, such as LiClO.sub.4, LiPF.sub.6, or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li.sup.+ ion mobility at conductivities in the range of about 10.sup.-3 S/cm.
Although serving well in this role of ion conductor, these separator elements unfortunately comprise sufficiently large solution-containing voids that continuous avenues may be established between the electrodes, thereby enabling lithium dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated manganese oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Although providing efficient power sources, these lithium-ion cells do not readily attain the capacity provided by lithium metal electrodes.
Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO.sub.4, LiPF.sub.6, or the like. A range of practical ionic conductivity, i.e, over about 10.sup.-5 to 10.sup.-3 S/cm, was only attainable with these polymer compositions at ambient conditions well above room temperature, however. Some improvement in the conductivity of the more popular poly(ethylene oxide) compositions has been reported to have been achieved by radiation-induced cross-linking (U.S. Pat. No. 5,009,970) or by meticulous blending with exotic ion-solvating polymer compositions (U.S. Pat. No. 5,041,346). Each of these attempts achieved limited success due to attendant expense and restricted implementation in commercial practice.
Some earlier examinations of poly(vinylidene fluoride) polymers and related fluorocarbon copolymers with trifluoroethylene or tetrafluoroethylene revealed enhancement of ionic conductivity by a simple incorporation of lithium salts and organic solvents which are compatible with both the polymer and salt components. This work by Tsuchida et al. (Electrochimica Acta, Vol. 28 (1983), No. 5, pp. 591-595 and No. 6, pp. 833-837) indicated, however, that the preferred poly(vinylidene fluoride) compositions were capable of exhibiting ionic conductivity above about 10.sup.-5 S/cm only at elevated temperatures, reportedly due to the inability of the composition to remain homogeneous, i.e., free of deleterious salt and polymer crystallites, at or below room temperature. Such limitations apparently led to the abandonment of attempts to implement these compositions in practical applications, such as rechargeable battery cells.
In our earlier investigations which resulted in the inventions claimed in the above-noted related applications, strong, flexible polymeric electrolytic cell separator membrane materials were discovered which readily retain electrolyte lithium salt solutions and remain functional over temperatures ranging well below room temperature. These electrolyte membranes were employed either in the usual manner as separator elements with mechanically assembled battery cell components or in composite battery cells constructed of successively coated layers of electrode and electrolyte compositions. In each of these implementations, however, the polymeric electrolyte/separator elements often contained the lithium electrolyte salts at the time of cell assembly and, due to the hygroscopic nature of those salts, thus necessitated extraordinary environmental conditions during cell assembly.
The present invention provides a manner of utilizing these improved polymeric electrolyte membrane and electrode compositions which substantially eliminates the need for special environmental controls during cell manufacture. Further, the present battery structure with its bonded layers requires less electrolyte, which in previous battery constructions was in part wasted in large voids, thereby yielding a more economical and versatile battery cell product.